Analysis of the formation of SiC and Si3N4 precipitates during directional solidification of multicrystalline silicon for solar cells

Directional solidification by the Vertical Gradient Freeze method (VGF) and related technologies is the most important process for the industrial production of multi-crystalline silicon for photovoltaic applications. A problem in this crystallization process is the formation of SiO2, SiC and Si3N4 precipitates. These defects are reducing the efficiency of solar cells and should therefore be avoided. The origin of the precipitates is assumed to be contamination of the silicon melt from graphite parts in the furnace and from the Si3N4 coating of the SiO2 crucible. However, the mechanisms of the formation of the precipitates are not well understood. The goal of this work is to study the C-, N-, and O-distributions in the silicon ingots and to correlate it to the occurrence of precipitates as well as to the growth conditions. Therefore, multi-crystalline silicon ingots with a diameter of 6 cm and a length of 4-5 cm were grown in Si3N4 coated silica crucibles in a laboratory-scale VGF facility. The experimentally obtained distributions of the impurities is explained using numerical simulations. Entnommen aus <a_href="http://www.fiz-technik.de/db/b_tema.htm" target="_blank">TEMA</a>